I want to put up eight three inch square by .120" thick posts twenty feet long, about two foot into concrete piers. Four of them would form a 7' square in the center, the other four would form a larger square of about
15". I want to stand them on end. This is for a hot tub/observation platform.
Where can I find the compression strength of that tubing? I'm sure this is going to be way strong enough, as the spa is about 300 gallons. So, that's a little under 3,000# for spa and water. There would be multiple bracing on the posts, both to each other, on the horizontal and on the diagonal. I believe a simple box with an X in the middle would give it sufficient strength to keep any lateral movement to a minimum. 1/4" would be over double the price, and overkill, IMHO.
If you haven't bought your steel yet, I'd consider bumping it up to .180" wall (3/16").
The issue won't be strength, it will be buckling and also corrosion. If it were my design, I'd plan it to bolt together and take all the pieces to be hot dip galvanized, and then build it with galvanized parts. But steel columns fail by buckling, and the way to avoid buckling is by reinforcing in your design.
Pure compression strength of a tube shorter than Euler's formula predicts is the cross sectional area times the compressive strength. Using 55kpsi, any one post is way more than you need. For steel, Euler's formula predicts failure by buckling for anything over length divided by diameter ratio of 89 to 1. You are right near that.
So the answer is you do not have a vertical compression due to weight problem. You have a lateral bracing and sway problem. Without looking at the actual layout and sizing of all the members, I wouldn't even hazard a guess what it would do.
I'd suggest that you should look at the classic cross country power l> I want to put up eight three inch square by .120" thick posts twenty feet
3000 lbs + 6 burly friends + beer :-)), 20 feet up in the air, requires engineering.
Is your area prone to storms, ice build-up, earthquakes, etc.? Then you would have to consider that, also. What are soil conditions like? Sand, mud, bed rock? Will this tower be free-standing or be attached to an existing structure?
I take it that the four outer posts are to support a surrounding deck? If so, consider a design with four posts only: say 4" x 4" x ? or 6" x 6" x ?. ( My GUESS is that it would take 6" x 6" x ?? for this app.) These four posts would support the tub AND a cantilevered deck. Don't forget the safety railing!!
While column buckling failure is ONE consideration, other modes of failure are torsional collapse (the tower twists during collapse), and tower shear failure (the structure moves sideways during collapse, legs "hinging" at the base). Both of these modes of failure are resisted by shear (diagonal) bracing.
Buckling failure is wrapped up in the formula Kl/r (slenderness ratio) where "K" is a factor depending on the column end conditions, "l" is the UNBRACED length of the column, and "r" is the section's radius of gyration. Most structural design handbooks have listings of Kl/r ranging from a low of 20 to a high of 200 with a corresponding allowable compressive stress in the steel, which varies only a little with the yield strength of the steel.
In my jurisdiction (Ontario, Canada) such a structure would require stamped and certified drawings by a licensed Professional Engineer to get it past the city building department.
A year or so ago a fellow called me because he wanted to put a hot tub on an extension to an existing deck. The city building department told him it would have to be "engineered" with stamp and signature of P.Eng. on the dwgs. He nearly choked when I explained to him what he was in for. And his deck was only 3' or 4' off the ground!
Put footers with galvanized J-bolts and baseplates - Rust Happens. Baseplates and bolts means it's repairable, even if you have to set up temporary columns and jacks to take the load while you rebuild it.
If you sink the posts straight into the concrete, you can't see internal rust happening, almost up to the point where it fails catastrophically and drops the entire deck, hot tub and all. You might get a short warning if you inspect it often and spot a rust perforation through the side of the tubing, but do NOT count on it.
I change out rusted through light standards weekly - the bolt-base ones are easy, take out the old hole and put in the new.
The direct burial ones are a Pain In The Ass, you have to get out the jackhammer and carefully take apart the old concrete footing to get it out and pour the new one. Inevitably they have three or four power conduits homing on the old pole base, so you can't just rip it out with a tractor.
An earlier poster suggested copying the base of a high-voltage transmission tower.... This is a good idea provided that you keep the same taper and bracing. The base you can adjust to suit the platform size on top.
Pouring concrete footings with anchor bolts is also an excellent idea for all the reasons stated above. It also allows small adjustments to be made in the verticality of the tower.
Perhaps there is a fire tower in your area you could copy if there are no high-voltage transmission lines.
Angle iron is much cheaper than hollow structural section (HSS), and you could bolt things together with structural Grade 5 bolts.
I think is a very doable project and I hope you post some photos with bathing beauties no less!
I have always had a fascination with observation towers, and perhaps the earliest ones I was ever on (I'm 60) were either fire observation towers or high power towers converted. My dad used to take us everywhere on summer vacation. He made it his duty in life to visit every snake pit an small town museum known to man. Good for us! Hooey, in most places kids were free. (We realized later the old man was just cheap, and didn't give a hoot of us getting a good education...... not really)
Fast forward about twenty years. I worked on a rig building crew in the Gulf of Mexico. I did it two years, and realized that I was not a rig builder. On that TV show, one of the Poole brothers I worked with is still doing it, but now he's a hoist operator and doesn't climb.
The standard derrick in those days was a Lee C. Moore 147. 147' with everything on it. Capable of lifting right at one million pounds. Today, that's spaghetti compared to these big topdrive rigs.
I still see a few of the Lee C Moore's around. There were two right here in Las Vegas. One is left, very near downtown Freemont Street. Ogden and Tenth approximately. I'd like to steal that one, but don't know who owns it, and couldn't afford it right now.
I could get one fabbed out of Houston, just up to the top of the Vee door, or maybe one leg higher. For them, it would be a slam dunk. Prepunched and hot galvanized dip, guaranteed to survive even in the Gulf of Mexico. But we're talking lots of bucks and those morons at Lotto still can't get the numbers right.
But yes, I will keep my eye out for a tower of that style. I want one for the cabin, too, but not for a hot tub. There's below zero weather there in the winter, and freezing about seven months otherwise. There's a new Rocky Mountain Power yard that opened just down the freeway and they're already stocking broken poles in there, I assume from car crashes. Guys I know have bought them pretty cheap. Maybe they have crashed metal towers, too.
Poles are harder to work with than the Erector set towers as to connecting. With the towers, if you get those first legs right, the rest goes up pretty easy and quick with knowledgeable help. All you need is a decent capstan. We used to use the one on the end of the drawworks. And a four strand 1" manila line.
I was thinking of square tube, as it is readily available, and easy to work with. Will look around on the tower idea, though.